A frequent metrology task in precision engineering is the surface texture analysis of nominally cylindrical areas, including bearing and sealing surfaces on rotating shafts. The machining process can leave signatures, intended or unintended, of the turning, grinding or honing process. These signatures often include groove bands, or more generally, a dominant direction or ensemble of direction for texture marks. The functional behavior of sealing surfaces and bearings can be strongly dependent on the dominant texture direction with respect to the axis of rotation of a machined part. This is for example the case for twist or machining lead angle, hereafter referred to as “lead angle,” which characterizes the orientation of the strongest texture direction with respect to the rotation axis. Following common usage in sealing surface characterization, and in analogy with terminology for screws and gears, the lead angle may be calculated from the arctangent of the axial advance of the nominally helical structure of the surface texture during one complete turn divided by the circumference of the one complete turn.
A variety of methods have been developed for measuring the lead angle. For example, a traditional mechanical means for detecting measuring lead angle involves the suspended weight or thread method, as detailed for example in the Rubber Manufacturers Association—Oil Seal Technical Bulletin—Shaft Requirements for Rotary Lip Seals standard (“RMA OS-1-1”) from 2004. Other methods rely on mechanically-contacting stylus measurements, as detailed, for example, in “Measurement and Evaluation Method for the Assessment of Lead-Reduced Dynamic Sealing Surfaces,” Mercedes-Benz Engineering Standard MBN 31 007-7, edition 2009-04.